Palladium(II) Complexation by p-Cyclophane Receptors. A Solution and Solid State Study Andrea Bencini,* ,² Antonio Bianchi,* ,² Vieri Fusi, ‡ Claudia Giorgi, ² Piero Paoletti,* ,² Jose′ Antonio Ramirez,* ,§ and Barbara Valtancoli ² Department of Chemistry, University of Florence, Via Maragliano 75/77, 50144 Florence, Italy, Institute of Chemical Sciences, University of Urbino, Italy, and Department of Chemistry, University of Valencia, C/Dr. Moliner 50, 46100 Burjassot, Valencia, Spain ReceiVed September 9, 1998 The coordination properties of the macrocyclic ligands 1,4,7,16,19,22-hexamethyl-1,4,7,16,19,22-hexaaza[9.9]- p-cyclophane (L1) and 1,4,7-trimethyl-19,22,28,31-tetraoxa-1,4,7,14,23-pentaaza[9.25]-p-cyclophane (L2) have been studied by means of potentiometric and 1 H and 13 C measurements in aqueous solution. L1 is composed of two equal triamine binding units connected by p-phenylene spacers. L2 presents a similar molecular architecture, a triamine moiety of L1 being replaced by a cyclic N 2 O 4 binding unit. L1 can form both mono- and dinuclear complexes in aqueous solution, while L2 gives only mononuclear species. The potentiometric data indicate that in the L1 dinuclear complex each metal is coordinated by a triamine moiety. In the L2 mononuclear complex the Pd(II) ion is coordinated by the N 3 unit. The N 2 O 4 moiety does not show any binding ability toward Pd(II), but exhibits a high tendency to protonate. These solution data are confirmed by the crystal structures of [Pd 2 Cl 2 L1](ClO 4 ) 2 ‚H 2 O (a) and [PdClL2H 2 (H 2 O)](ClO 4 ) 3 (b). In complex a, each Pd(II) ion is four coordinated by the three amine groups of the triaza moiety and a chloride anion, in a square planar geometry. In the mononuclear complex b, the metal is coordinated by the N 3 moiety, with a coordination environment almost equal to that found in complex a. The N 2 O 4 moiety is diprotonated and encapsulates in its cavity a water molecule, held by a hydrogen-bond network. The solution structures of the L1 and L2 complexes have been studied by means of 1 H and 13 C NMR measurements. The analysis of the NMR data reveals that the dinuclear L1 complex and the L2 mononuclear one show structural features in solution almost equal to those found in the solid state. Solution and solid state data indicate that the Pd(II) complexation gives a marked stiffening of the macrocyclic structures. Introduction Macrocycles containing two binding polyamine subunits linked by two chains are known to form dinuclear complexes and can be used to force two metal ions at almost fixed dis- tances. 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